Switching amplifier, also known as class-D amplifier, is advantageous in efficiency and thereby has benefits when compared to class-AB amplifier. As shown in FIG. 1, a typical switching amplifier 100′ including a low-pass filter (LPF) comprises an H-bridge circuit composed of switching transistors M1, M2, M3 and M4, and a control circuit 102′ in response to an input signal Vi to switch the transistors M1, M2, M3 and M4 to generate a pair of output signals OUTP and OUTN. If the switching amplifier 100′ is directly connected to an inductive load 106′ such as a coil-type speaker, due to the output signals OUTP and OUTN in opposite polarity, there will be a large switching current flowing through the load 106′, causing severe electro-magnetic interference (EMI) problems and having the load 106′ to bear significant power dissipation outside of the audio frequency band. It is thus necessary to filter out the signal component outside of the audio frequency band from the output signals OUTP and OUTN by the LPF 104′ before they are applied to the load 106′. FIG. 2 shows a waveform diagram of the output signals OUTP and OUTN of the switching amplifier 100′ at different-level input signal Vi's, in which waveform 108′ represents the output signal OUTP and waveform 110′ represents the output signal OUTN. When the input signal Vi=0, the duty cycles of the output signals OUTP and OUTN both are 50%, and when the input signal Vi>0, the duty cycle of the output signal OUTP increases, and the duty cycle of the output signal OUTN decreases. Contrarily, when the input signal Vi<0, the duty cycle of the output signal OUTP decreases and the duty cycle of the output signal OUTN increases. In other switching amplifiers, it may be the case instead that, when the input signal Vi>0, the duty cycle of the output signal OUTP is less than 50%, while the duty cycle of the output signal OUTN is greater than 50%, and when the input signal Vi<0, the duty cycle of the output signal OUTP is greater than 50% while the duty cycle of the output signal OUTN is less than 50%.
However, introducing the LPF 104′ results in cost raise and efficiency drop when implementing a switching amplifier, and therefore it is proposed filterless switching amplifier. Under appropriate switching control, for instance disclosed in U.S. Pat. Nos. 6,211,728 and 6,262,632, a filterless switching amplifier may retain the advantages in efficiency. Additionally, the EMI issue is also important in the design of a filterless switching amplifier. The present invention is directed to a high efficiency and low EMI switching amplifier and a control method thereof.